Apparatus for controlling the energization of a load

ABSTRACT

A plurality of condition responsive impedances, such as PTC thermistors are each connected to reference impedances to form voltage dividers which form half of a bridge circuit. The voltage divider junctions are coupled through diodes to the base of an NPN transistor in the detector of the bridge circuit, the emitter-collector circuit of the transistor being connected across the gate cathode circuit of a silicon controlled rectifier which is used to control the energization of a serially connected relay coil. In the absence of a predetermined condition, such as excessive temperature in a load, the silicon controlled rectifier is gated on each half cycle thereby allowing current to flow through the relay coil to energize the load. On the occurrence of the predetermined condition, i.e., excessive temperature, the impedance value of the condition responsive impedances change and cause conduction of the transistor which shunts current away from the gate of the silicon controlled rectifier thereby deenergizing the relay coil. Differential between the temperature at which energization and deenergization of the system occurs is provided by a reset impedance with contacts coupled thereacross, the contacts being closed when the relay coil is energized. Bypass diodes are connected across the reset and reference impedances to increase the temperature differential and to effect a longer delay before reenergization.

United States Patent Strachan I APPARATUS FOR CONTROLLING THE ENERGIZA'IION OF A LOAD Primary ExaminerJ. D. Miller Assistant Examiner--Harry E. Moose, Jr.

Attorney- Harold Levine, Edward J. Connors, Jr. and John A. Haug et al.

[57] ABSTRACT A plurality of condition responsive impedances, such as June 19, 1973 PTC thermistors are each connected to reference impedances to form voltage dividers which form half of a bridge circuit. The voltage divider junctions are coupled through diodes to the base of an NPN transistor in the detector of the bridge circuit, the emitter-collector circuit of the transistor being connected across the gate cathode circuit of a silicon controlled rectifier which is used to control the energization of a serially connected relay coil. In the absence ofa predetermined condition, such as excessive temperature in a load, the silicon controlled rectifier is gated on each half cycle thereby allowing current to flow through the relay coil to energize the load. On the occurrence of the predetermined condition, i.e., excessive temperature, the impedance value of the condition responsive impedances change and cause conduction of the transistor which shunts current away from the gate of the silicon controlled rectifier thereby deenergizing the relay coil. Differential between the temperature at which energization and deenergization of the system occurs is provided by a reset impedance with contacts coupled thereacross, the contacts being closed when the relay coil is energized. Bypass diodes are connected across the reset and reference impedances to increase the temperature differential and to effect a longer delay before reenergization.

8 Claims, 3 Drawing Figures v l "9* w A AVJTAIA 196? 0/ 175 L7 M A I o l Eng RY/LEEZ/DJ R4 1.6 08 l w .93 l M W I E07 J3 I i d f ir ,1 J2 506 24 s i 1 m 5 l 22 R lW/A l 09 W D {La m; Q We Patented June 19, 1973 2 Sheets-Sheet 1 Pa tented' June 19, 1973 2 Sheets-She et 2 l I A mari- TEMPE/47.470295 N mm m f v i Q6 fixkkkbwww r APPARATUS FOR CONTROLLING THE ENERGIZATION OF A LOAD This invention relates to control apparatus and more particularly to apparatus for controlling the energization of a load based on a variable condition. A similar system is described and claimed in U.S. Pat. No. 3,329,869. In that system a plurality of conditionresponsive voltage dividers are connected in parallel across a voltage source, each voltage divider including a condition-responsive impedance element and a reference impedance element connected in series therewith. A single relay means or current switch means for controlling the flow of electric power to a load device is operated under the control of a plurality of transistors having an output circuit interconnected with a control circuit for the relay means and an input circuit for controlling the conductive state of the output circuit. The input circuits of the transistors are interconnected with respective voltage dividing circuits, whereby each of the voltage dividing circuits functions independently to exercise a control of the relay means. In one of the embodiments the relay means includes a silicon controlled rectifier and the transistors, the emitter-collector circuits of which are connected in parallel across the gatecathode circuit of the silicon controlled rectifier.

As explained in the above. patent, such a system is useful inproviding, inter alia, thermal protection for electric motors, particularly polyphase AC. motors such as compressor motors for refrigeration and airconditioning apparatus, where the temperature of the windings is sensed at several locations by sensors having a rapid response to temperature changes. The system causes the motor to be immediately deenergized if the temperature of the portions contiguous any one of these locations rises above a predetermined temperature.

Although the above system is effective for many such applications there are certain inherent characteristics which limit its flexibility and usefulness. For instance where cost is an important factor it is desirable to provide a less expensive circuit, for instance, one having fewer of the higher cost circuit elements. In applications where long life is required and having many cyclical operations the above system is not fully satisfactory since the relay contactstend to chatter for a short time on trip, that is, the relay chatters upon deenergization of the motor due to excessive heat generation. It will readily be seen that chattering for even a short time would have a serious effect on contact life. For instance, if the contacts chattered only four times on reset such chattering would cut down contact life by a factor of four. Another limitation is an application in which it is necessary to maintain a specific reset temperature over a long period of time. Although it is not understood why, the reset temperature of the above system tends to drift with age. Yet another limitation is in applications where a relatively large temperature differential is required, that is, the difference between the temperature at which the system operates to deenergize a motor and that at which it permits the motor to be reenergized.

It is therefore an object of the invention to provide control apparatus for controlling a load device such as a motor, which is inexpensive yet reliable in operation, one which has a wide temperature differential and whose reset temperature does not drift.

Another object is the provision of such apparatus which is long lived, one whose relay contacts do not chatter.

Another object is the provision of control apparatus which obviates the limitations of the prior art systems.

Other objects and features of the invention may be more readily understood from the following detailed description and appended claims when read in conjunction with the accompanying drawings.

Briefly, the invention relates to a control in which one or more voltage dividers each of which forms half of a bridge circuit and comprises a reference impedance and a serially coupled condition responsive impedance, forms a voltage divider junction therebetween, which junction is connected through a diode to the base of a single transistor in the detector of the bridge circuit. The transistor is arranged so that when it is in the conductive state it shunts triggering current away from the gate of a silicon controlled rectifier which is serially connected to a relay coil thereby controlling the energization of the coil. A differential impedance is shunted by contacts which are closed when the relay is energized to create a temperature differential between deenergization and reenergization. Bypass diodes are connected across the reference and differential impedances in order to subject the condition responsive impedances in the form of PTC thermistors to essentially all of the available voltage on alternate half cycles thereby keeping them in the self heated, high resistance mode. I

In the accompanying drawings:

FIG. 1 is a schematic diagram of a preferred embodiment of the present invention;

FIG. 2 is a resistance versus temperature curve for a typical sensor employed in the present invention; and

FIG. 3 is a temperature versus time curve for such a sensor and a load.

Referring now to FIG. 1 the preferred embodiment comprises a step up or step down transformer 12 depending upon the voltage source having a primary.

winding 14 to which are connected lines L1 and L2 for supplying power from a conventional A..C. source. Transformer 12 is provided with a secondary winding 16 having an intermediate tap 18. Tap 18 is preferably a center tap so that the voltages of the two half windings are matched. Lines L3 and L4 are connected to opposite ends of secondary winding 16 while line L5 is connected to the center tap 18.

Relay coil RYl is connected between one side of winding 16 by line L3 and the center tap 18 by line L5. Also serially connected thereto is a silicon-controlled rectifier SCRl. Preferably a diode D1 is placed in line L5 to protect SCRl from high voltage negative transients.

A plurality of voltage dividers 22, 24, 26 are connected in parallel across secondary 16. Each comprises a condition responsive impedance 8], S2, S3 respectively serially connected to a reference impedance R1, R2, R3 respectively and form respective junctions J1, J2, J3 therebetween. Condition responsive impedances S1, S2 and S3 are preferably formed of a steeply sloped positive temperature coefficient (PTC) of resistivity thermistors such as barium titanate doped with lanthanum. Although three voltage dividers are shown it will be understood that the number employed is a matter of choice depending on the application for which the circuit is to be used. In some instances it may be desirable to have only a single divider or channel while in other instances it may be desirable to have two or even more than three.

Junctions J1, J2 and J3 are connected to form an or gate to the base b of an NPN transistor Q1 through respective diodes D2, D3 and D4 and in series with current limiting resistor R4.

Resistor R5 interconnects the anode and gate electrodes of rectifier SCRl, providing triggering current to fire the rectifier when the forward direction of the AC. voltage is applied across the anode-cathode circuit.

The emitter-collector output circuit of transistor Q1 is connected across the gate-cathode circuit of the siliconcontrolled rectifier SCRl. Resistor R6 is connected between the collector and the gate of the rectifier SCRl to obtain the correct impedance level. Capacitor C 1 connected across the gate-cathode circuit of rectifier SCRl decreases line transient sensitivity while resistor R7 also connected across the gate-cathode circuit of rectifier SCRl is effective to minimize the effects of variation in triggering current due to the variation among silicon-controlled rectifiers and due to temperature variations.

Since the silicon-controlled rectifier is a device which permits current to flow in a single direction diode D5 is connected across relay winding RYl to prevent the relay contacts from dropping out each half cycle by permitting the inductively stored current to free-wheel during alternate half cycles.

A reset or differential impedance R8 is serially connected between the voltage dividers and the line L4 side of secondary winding 16. Contacts RYlA, controlled by relay coil RYl, shunt impedance R8 and are closed when coil RYl is in the energized condition.

For a purpose to be described below, reference impedances R1, R2, R3 and reset impedance R8 are shunted by respective diodes D6, D7, D8 and D9.

Relay coil RY] also controls a second set of contacts RYlB in lines L6, L7 which may be part of the coil circuit of a contactor which connects a load (e.g., motor) to a power source.

Operation of the circuit will be described by assuming that the instantaneous polarity of the potential of line L3 is positive unless otherwise stated. The voltage divider 22 forms a bridge with the tapped winding 16 which has a detector circuit including diode D2, resistor R4, transistor Q1 and diode D1. As mentioned above S1 is a PTC thermistor which may be embedded in the windings ofa motor to be protected. During normal operation voltage is supplied by transformer 12 from a conventional A.C. source. Current flows through bridge 16/22 with essentially no current flowing in the detector circuit since the impedance of sensor S1 is lower than that of reference impedance R1. Triggering current is supplied to silicon-controlled rectifier SCRl through resistor R5 turning the rectifier on each positive half-cycle. Current flowing through relay coil RYl' maintains contact RYlB closed and hence maintains energization of the load. Contacts RY1A are also maintained closed by current flow through relay coil RYl thereby shunting reset impedance R8. If the winding temperature increases beyond a predetermined safe limit the resistance of thermistor S1 increases causing the voltage at junction J1 to increase to a potential higher than at tap 18 thereby causing current to flow into the base of transistor Q1 causing it to conduct current in the collector-emitter circuit. When O1 is in the conductive state it shunts current around the gate-cathode circuit of silicon-controlled rectifier SCRl so that it is not turned on at the beginning of each half cycle and relay coil RYl is then deenergized and contacts RYlA and RYlB open.

When contacts RYlA open impedance R8 is placed in the circuit which further increases the voltage at junction J1 thereby insuring positive action of the silicon-controlled rectifier SCRI and relay RYl avoiding chattering of contacts RYlB. As noted above, during normal operation contacts RYlA are closed. Provision of contacts which are in the closed condition when relay coil RYl is energized results in immediate inclusion of impedance R8 upon deenergization of relay coil RYl and consequent increase in voltage at junction J1.

In the patent mentioned above, the function of impedance R8 is performed by three resistors which are on the opposite side of the detector. When the resistors are in this position, they must be switched out of the circuit upon deenergization of relay. This is done by a set of normally closed contacts. The movable contact must travel the total length of its traverse before closure is made. Located as shown in FIG. 1 of the present invention, impedance R8 is switched into the circuit upon deenergization of the relay by the opening of a set of normally open contacts. The movable contact need travel only a very short distance from the fixed contact to break the bypass circuit and quickly switch the impedance R8 into the circuit. This difference in time could well cover several cycles and hence is important in insuring that chattering is eliminated. Upon cessation of current in relay winding RYl contacts RYlA open immediately. The inclusion of impedance R8 causes a differentialin temperature between that temperature at which the relay is deenergized and the contacts opened and that temperature at which the relay is again energized and the contacts reclosed. This differential permits the motor to cool before restarting can be attempted. As seen in FIG. 2 the slope of the resistivity versus temperature curve is very steep. While this is desirable to obtain a specific predetermined trip temperature is also has the effect of limiting the temperature differential between the trip and reset points.

The operation of condition responsive impedances S2 and S3 are identical to that of S1. Diodes D2, D3 and D4 prevent interaction among the channels so that the particular resistance level of one of the thermistors S1, S2, S3 has virtually no effect in the temperature at which either the other two thermistors will trip the circuit.

Bypass diodes D6, D7, D8 and D9 each half cycle in.

the negative direction, place essentially all of the volt age from the secondary winding 16 across the termistors S1-S3 which cause them to self heat so that upon tripping of the circuit there will be a longer delay before the circuit resets. That is, when line L3 is negative with respect to line L4, the impedances are bypassed throwing essentially all of the secondary winding voltage across the thermistors Sl-S3 thereby causing them to heat keeping their resistance higher, this raises the voltage level at the anodes of diodes D2-D4 which tends to maintain transistor Q1 on and siliconcontrolled rectifier SCRl off for a longer period of time thereby permitting the load to cool even more. The effect of this ay be seen in FIG. 3 which shows the temperature versus time curves, upon tripping of the circuit, curve 30 for a thermistor S1 as used in the circuit of the present invention; curve32 for a load, e.g.,

motor winding; and curve 34 for a thermistor as used in the prior art circuit reference supra. It will be readily apparent that an improved temperature differential, Al compared to A2, is achieved by causing the thermistors to be heatedto a higher temperature thereby enabling the motor winding more time to cool. Although the same voltage is placed across the thermistors when the relay is closed, the trip temperature is not significantly effected since the resistance of the thermistors near the trip point is much higher than when near the reset point.

As mentioned supra, the reset resistance of the prior art circuit contained in US. Pat. No. 3,329,869, was foundto have a tendency to drift. It can be seen from FIG. 2 that any drift in resistance from nominal reset resistance R in. and around the reset region, has a magnified effect on the temperature due to the shallowness of the curve in that region. The circuit of the present invention does not have this tendency. Some drift in trip resistance of the circuit in the trip region from nominal trip resistance T is noted, however this is not significant because the resistance versus temperature curve of the sensor in that region is so much steeper that only a small deltatemperature results. In practice thisdelta is found to be less than two degrees Centigrade.

Thus, it will be seen that the present invention provides an improved control apparatus, one which has no reset temperature drift, is significantly less expensive than the prior art circuit as described above in that only one set of reset contacts and only one transistor are required. Further, it has been found that a less expensive plastic encapsulated silicon transistor can be used in the circuit of the invention while the more expensive standard germanium transistor is required in the prior art circuit. A silicon transistor offers another advantage in that it provides a greater temperature stability than does a germanium transistor. Further, contactor chatter has been completely eliminated and effective temperature differential has been increased. In view of the above, it will be seen that the several objects of the invention have been achieved and other advantageous results attained.

As various changes could be made in the above construction and circuit without departing from the scope of the invention, it is intended that all matter contained in'the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. Control apparatus responsive to a plurality of variable. conditions for controlling a load device comprismg: I

a plurality of voltage dividers each of which includes a condition-responsive impedance element serially connected with a reference impedance element, each of the voltage dividers having an electrical junction between the condition-responsive and reference impedance element thereof and respective bypass diodes connected across each reference impedance element;

a voltage source, the voltage dividers being con- I nected in parallel across the source;

relay means for controlling the flow of electric power to the load device, the relay means including a control circuit;

and an electronic element having anoutput circuit and an input circuit for controlling the conductive state of the output circuit, the output circuit of the electronic element being interconnected with the control circuit of the relay means, the operation of the relay means being dependent upon the conductive state of the electronic element, the input of the electronic element being coupled to each of the voltage divider junctions to control the conductivity of the output circuit, whereby the conditionresponsive elements function to exercise control over the load device.

2. Control apparatus-according to claim 1 in which the relay means includes a gated power switch.

3. Control apparatus according to claim 2 in which the electronic element is a transistor and the gated power switch is a silicon-controlled rectifier, the emitter-collector circuit of the transistor is connected across the gate-cathode circuit of the silicon-controlled rectifier.

4. Control apparatus according to claim 3 in which each voltage divider junction is coupled to the base of the transistor through separate diodes.

5. Control apparatus responsive to a plurality of variable conditions for controlling a load device comprising:

voltage source including a transformer winding having an intermediate tap;

a relay coil serially coupled to one end of the transformer winding and the intermediate tap,

a silicon-controlled rectifier serially connected to the relay coil,

a plurality of voltage dividers each comprising a condition responsive impedance and a serially coupled reference impedance, a voltage divider junction formed between respective condition responsive impedances and reference impedances,,

the plurality of voltage dividers coupled in parallel across the transformer winding;

means providing triggering current to the gate of the silicon-controlled rectifier;

an NPN silicon transistor having an emitter-collector circuit coupled across the gate-cathode circuit of the silicon-controlled rectifier, each voltage divider junction coupled to the base terminal of the transistor through respective diodes; and

reset means comprising a reset impedance coupled between the other end of the transformer winding and the voltage dividers, contacts coupled across the reset impedance, the contacts being controlled by the relay winding so that when the relay winding is energized the contacts are closed, and bypass diodes connected across the reset impedance and the reference impedances.

6. Control apparatus responsive to a variable condition for controlling a load device comprising:

a voltage source comprising a transformer winding and tap intermediate the ends thereof;

a relay coil and a power switch serially connected between one end of the transformer winding and the p! a voltage divider connected across the transformer winding and forming a bridge circuit therewith, the voltage divider comprising a condition responsive impedance and a reference impedance and a voltage divider junction being formed therebetween;

voltage responsive means in the detector of the bridge circuit coupled to the voltage divider junc- 1 tion and the power switch for controlling the energization of the power switch in response to the voltage level at the junction; and

reset impedance means connected between the other end of the transformer winding and the voltage divider, a set of contacts connected across the reset impedance means, the contacts controlled by the relay coil such that the contacts are closed when the relay coil is energized and respective bypass diodes connected across the reference impedance and the reset impedance.

7. Control apparatus according to claim 6 in which the condition responsive impedance is a PTC thermistor.

8. Control apparatus responsive to a variable condition for controlling a load device comprising:

a voltage source comprising a transformer winding and tap intermediate the ends thereof;

a relay coil and a power switch serially connected between one end of the transformer winding and the tap,

a voltage divider connected across the transformer winding and forming a bridge circuit therewith, the voltage divider comprising a condition responsive impedance and a reference impedance and a voltage divider junction being formed therebetween;

voltage responsive means in the detector of the bridge circuit coupled to the voltage divider junction and the power switch for controlling the energization of the power switch in response to the voltage level at the junction; and

reset impedance means connected between the other nected across the resetimpedance. 

1. Control apparatus responsive to a plurality of variable conditions for controlling a load device comprising: a plurality of voltage dividers each of which includes a condition-responsive impedance element serially connected with a reference impedance element, each of the voltage dividers having an electrical junction between the condition-responsive and reference impedance element thereof and respective bypass diodes connected across each reference impedance element; a voltage source, the voltage dividers being connected in parallel across the source; relay means for controlling the flow of electric power to the load device, the relay means including a control circuit; and an electronic element having an output circuit and an input circuit for controlling the conductive state of the output circuit, the output circuit of the electronic element being interconnected with the control circuit of the relay means, the operation of the relay means being dependent upon the conductive state of the electronic element, the input of the electronic element being coupled to each of the voltage divider junctions to control the conductivity of the output circuit, whereby the condition-responsive elements function to exercise control over the load device.
 2. Control apparatus according to claim 1 in which the relay means includes a gated power switch.
 3. Control Apparatus according to claim 2 in which the electronic element is a transistor and the gated power switch is a silicon-controlled rectifier, the emitter-collector circuit of the transistor is connected across the gate-cathode circuit of the silicon-controlled rectifier.
 4. Control apparatus according to claim 3 in which each voltage divider junction is coupled to the base of the transistor through separate diodes.
 5. Control apparatus responsive to a plurality of variable conditions for controlling a load device comprising: voltage source including a transformer winding having an intermediate tap; a relay coil serially coupled to one end of the transformer winding and the intermediate tap, a silicon-controlled rectifier serially connected to the relay coil, a plurality of voltage dividers each comprising a condition responsive impedance and a serially coupled reference impedance, a voltage divider junction formed between respective condition responsive impedances and reference impedances,, the plurality of voltage dividers coupled in parallel across the transformer winding; means providing triggering current to the gate of the silicon-controlled rectifier; an NPN silicon transistor having an emitter-collector circuit coupled across the gate-cathode circuit of the silicon-controlled rectifier, each voltage divider junction coupled to the base terminal of the transistor through respective diodes; and reset means comprising a reset impedance coupled between the other end of the transformer winding and the voltage dividers, contacts coupled across the reset impedance, the contacts being controlled by the relay winding so that when the relay winding is energized the contacts are closed, and bypass diodes connected across the reset impedance and the reference impedances.
 6. Control apparatus responsive to a variable condition for controlling a load device comprising: a voltage source comprising a transformer winding and tap intermediate the ends thereof; a relay coil and a power switch serially connected between one end of the transformer winding and the tap, a voltage divider connected across the transformer winding and forming a bridge circuit therewith, the voltage divider comprising a condition responsive impedance and a reference impedance and a voltage divider junction being formed therebetween; voltage responsive means in the detector of the bridge circuit coupled to the voltage divider junction and the power switch for controlling the energization of the power switch in response to the voltage level at the junction; and reset impedance means connected between the other end of the transformer winding and the voltage divider, a set of contacts connected across the reset impedance means, the contacts controlled by the relay coil such that the contacts are closed when the relay coil is energized and respective bypass diodes connected across the reference impedance and the reset impedance.
 7. Control apparatus according to claim 6 in which the condition responsive impedance is a PTC thermistor.
 8. Control apparatus responsive to a variable condition for controlling a load device comprising: a voltage source comprising a transformer winding and tap intermediate the ends thereof; a relay coil and a power switch serially connected between one end of the transformer winding and the tap, a voltage divider connected across the transformer winding and forming a bridge circuit therewith, the voltage divider comprising a condition responsive impedance and a reference impedance and a voltage divider junction being formed therebetween; voltage responsive means in the detector of the bridge circuit coupled to the voltage divider junction and the power switch for controlling the energization of the power switch in response to the voltage level at the junction; and reset impedance means connected between the other end of the transformer winding and the voltage divider, a set of contacTs connected across the reset impedance means, the contacts controlled by the relay coil such that the contacts are closed when the relay coil is energized, and a bypass diode connected across the reset impedance. 